Data obtaining means repeatedly obtains acceleration data. Acceleration vector generation means generates first acceleration vector in accordance with first acceleration data obtained by the data obtaining means, and generates second acceleration vector in accordance with second acceleration data time-sequentially obtained by the data obtaining means following the first acceleration data. The cross product direction calculation means calculates a direction of a cross product between the first acceleration vector and the second acceleration vector. The swing direction identification means identifies a swing direction in which the input device is swung in accordance with the direction of the cross product.